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TopBand: Noise Phasing Units

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Subject: TopBand: Noise Phasing Units
From: (w8ji.tom)
Date: Sun, 10 Jan 1999 13:20:11 -0500
I can add a few things about these noise cancelers, since I spent some
quality time with them.

First, the 1025/1026 is a derivative of a unit I've used here for some
time. Unlike other similar units, the MFJ does NOT go through wild
amplitude changes as phase is varied. It has a nearly level amplitude as
phase is rotated, generally much less than 1 dB amplitude ripple. That is
very important, because the last thing you want to do is have to go back
and forth between gain controls and phase every time the phase  changes.

One design, despite claims, does not even rotate phase! Nulling noise is a
matter of luck! The phase controls changes only the level, and flips phase
180 degrees.

The primary drawback of the MFJ (and other units) is IMD performance and
noise floor. My unit uses $50 FET's (for low noise high IMD performance but
would cost nearly $1000 to market.

If IMD from out-of-band signals is a problem, the MFJ unit can be greatly
improved by adding a parallel L/C circuit across each "gain" pot, from high
end of each pot to ground. The component values are 2 uH in parallel with
3.35 NF (3350 uuF). (I use these components and others across each Beverage
matching system on the low Z side to keeps strong SW BC signals from
bothering my amplifiers. I've measured signal levels of three VOLTS from
some Beverages pointed at SW BC stations thousands of miles away!)   

This kills gain above 3 MHz and below 1.6 MHz, but this makes the unit much
better for topband use on big antennas.

None of these units work well for most storm QRN, unless the storm is a
narrow area of thunderstorms in one general direction. They will however,
be useful for peaking a signal by adding the desired signals from two
different antennas. That's almost always good for two or three dB extra

Anyone wanting a modified loaner unit for DXpeditions should contact me. I
have two band-switched loaner units that cover 160, 80, and 40 meters only.

73 Tom

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